The present invention relates to a process and a system for reducing leakage of LO to the RF-port (LLO) in a real system that converts the frequency of a first signal, i.e. the LO signal, by mixing the first signal with a second supplied signal SI to a third signal SO of a frequency that is different from the frequency of the first and second signal. More specifically the invention relates to a process and a system that are applied to suppress LLO in real image reject balanced passive mode transistor mixers. Further, the invention also relates to a balanced passive mode transistor mixer and an image reject balanced mixer to be used in said process and system.
In modern radio communication there is used a conversion from a first carrier wave frequency to a second frequency. For this conversion a frequency converter/mixer is employed. If the second signal is the IF and the third signal is the RF, the mixer is called an upconverter. Otherwise, if the second signal is the RF and the third signal is the IF, the mixer is called a downconverter. The mixer may be realised equally for these two modus of the mixer. This is the case for the kind of mixer discussed herein. For the upconverter, the IF is supplied and the RF is produced, while for the downconverter the RF is supplied and the IF is produced. In the following description, the upconverter configuration is used, but the present invention is not restricted to this embodiment.
Ideally the mixer functions so that it receives the signal which is to be convertedxe2x80x94IFxe2x80x94and a local oscillator signalxe2x80x94LOxe2x80x94and emits only one signalxe2x80x94RFxe2x80x94which has a frequency equal to the sum or the difference of the frequencies for IF and LO. However, practical limitations cause several other undesired signals also to be present together with RF. A so-called xe2x80x9cimagexe2x80x9dxe2x80x94IMxe2x80x94will be present (both xe2x80x9cthe sum signalxe2x80x9d and xe2x80x9cthe difference signalxe2x80x9d will come to the RF-gatexe2x80x94the one is desired, while the other is IM and consequently undesired). LO will leak out to the RF-port (Even if the mixer is used as a downconverter, the LLO is the LO leaking to the RF-port) In addition a series of other undesired signals named spurious signals, will arise in the mixer. The important ones can be made small and insignificant by employing as far as possible a linear mixing element and by allowing LO to be strong and dominating relative to IF and RF. The level of leaked LOxe2x80x94LLOxe2x80x94is proportional to the level of the supplied LOxe2x80x94SLO. The two dominating spurious signals will therefore be IM and LLO.
The prior art describes some approaches for reducing the LLO. A short review of these approaches is given in U.S. Pat. No. 6,343,211, as incorporated herein by reference.
U.S. Pat. No. 6,343,211 also describes a system and a process where two balanced mixers are combined in an image reject mixer configuration. The varying LOL from each of the balanced mixers are 90 degrees out of phase, and these two leakages (named LOL1 and LOL2 in the indicated application) are added before the RF-port, and the resulting leakage will be cancelled.
Applicants U.S. patent application Ser. No. 09/218,880 also describes a system and a process where two balanced mixers are combined in a image reject mixer configuration. The varying LOL from each of the balanced mixers are 90 degrees out of phase, and these two leakages (named LOL1 and LOL2 in the indicated application) are added before the RF-port, and the resulting leakage will be cancelled.
Yngve Thodesen, A resistive HEMT image reject upconverter with tunable LO supression. 30th ARMMS Conference Digest, 1999 describes two related approaches. The first approach, also disclosed in U.S. patent application Ser. No. 09/218,880, is a resitive HEMT mixer with no drain bias, and is cold. A gate bias Vg is usually applied. The LO is supplied to the gate, while the IF and RF exist at the drain. The LOL then depends on how the LO leaks through the HEMT This application is easy to implement and suitable for MMIC design. However, it is not very robust.
The first approach is limited by the fact that the LO-power needed to drive the mixer element into a wel-behaved modus simultaneously limits the dynamic in the regulations. The approach to this problem is to get parameters usable for the regulation that are not overlapping the ones used to control performance. These new parameters are provided by four PIN-diodes. This second approach is more robust, but the four PIN diodes make the solution expensive and not suitable for MMIC design.
The aim of the present invention is thus to develop a solution that combines robustness, simplicity and MMIC compatibility.
The purpose of the present invention is to minimise the limitations of prior art solutions and systems, by providing a process, system and mixers where passive mode transistors are utilised in mixers, and where these are arranged together with reflecting elements so that the unwanted LLO is reduced considerably. The system and the process according to the present invention can thus be employed where limitations in the components produce non-ideal conditions.
In accordance with the present invention there is provided a process for reducing leakage of LO in a non-ideal system that converts the frequency of a first signal, i.e. the LO signal, by mixing the first signal with a second supplied signal SI to a third signal SO of a frequency that is different from the frequency of the first and second signal, wherein
the first signal is divided via a number of couplers into a number of part-signals, preferably 4, which are phase-displaced relative to each other, and
that each such part-signal is led to its respective transistor there being partly absorbed, partly leaking through and partly reflected, and
that the part being absorbed by each transistor are mixed with the second signal SI and frequency-shifted, and
that the reflected signal from each transistor, while being adjusted by some regulating means, are transferred back to each transistor where a part of it is leaking through, and
that the frequency-shifted part-signals are added to a third signal via a number of couplers after the signals are phase-displaced relative to each other, and
that the regulating means being adjusted so that the LO-leakage signal caused by the reflection and the regulating means is cancelling the direct LO-leakage signal so that the total LO-leakage is reduced.
A preferred embodiment of the present invention relates to a process, wherein the IF-lines are arranged so that none of these crosses, wherein the signals from the two couplers are fed to the respective decoupling capacitors to make possible for the two outer filters to be placed in the same branches as the two inner filters respectively, so that an IF-structure with couplers may be placed together inside the mixer, while being connected to the outside of the mixer through filters, while utilising couplers that do not attenuate the IF-signal, and that a decoupling capasitor is placed after the last coupler to isolate the IF-signal from leaking to the RF-port.
Further, the invention also relates to a system for reducing the leakage of LO in a non-ideal system that converts the frequency of a first signal by mixing the first signal with a second supplied signal SI to a third signal SO of a frequency that is different from the frequency for the first and second signal, wherein the system comprises:
coupler means wherein the first signal is divided into a number of part-signals, preferably 4, which are phase-displaced relative to each other, and
transistor means whereto each signal is fed, there being partly absorbed, partly leaking through and partly reflected, and that the part being absorbed by each transistor are mixed with the second signal SI and frequency-shifted, and
regulation means wherein the reflected signal from each transistor being adjusted and transferred back to each transistor where a part of it is leaking through, and
coupler means wherein the frequency-shifted part-signals are added to a third signal, after the signals are phase-displaced relative to each other, and
that the regulating means being adjusted so that the LO-leakage signal caused by the reflection and the regulating means is cancelling the direct LO-leakage signal so that the total LO-leakage is reduced.
A preferred embodiment of this system is characterized in that the IF-lines are arranged so that none of these crosses, wherein decoupling capacitors are placed after the couplers to make possible for the two outer filters to be placed in the same branches as the two inner filters respectively, so that an IF-structure with couplers may be placed together inside the mixer, while being connected to the outside mixer through filters, while utilising couplers that do not attenuate the IF-signal, and that a decoupling capacitor is placed after the last coupler to isolate the IF-signal from leaking to the RF-port.
Further, the invention also relates to a balanced passive mode transistor mixer and an image reject balanced mixer.
Further, the invention also relates to a balanced passive mode transistor mixer employing a PIN-diode or a drain-side of a passive mode transistor as a tunable reflecting load and an image reject balanced mixer wherein the couplers are 90xc2x0 couplers preferred embodiments of the invention are given in the subclaims 2-12, 14-21, 23 and